1. Field of the Invention
The present invention relates to a method for the manufacture of (U,Pu)O.sub.2 mixed crystals as a preliminary stage for the fabrication of sintered nuclear fuel bodies with uranium- and plutonium-containing raw materials.
2. Background of the Invention
The nuclear fuel bodies are generally in pellet form and are customarily inserted into cladding tubes which then serve as fuel rods for the construction of fuel assemblies. These fuel assemblies serve for the generation of energy in the core of nuclear reactors, which usually operate according to the principle of thermal reactors or of fast breeder reactors. In these nuclear reactors, plutonium is formed in the nuclear fuel during the burnup of the fuel assemblies, which is recovered in reprocessing plants, and can and must be reused as nuclear fuel.
To achieve uniform heat production over the entire length of the fuel rods or, in other words, to avoid overheated places in the fuel rods of the core, it is necessary to mix uranium dioxide and plutonium dioxide so intimately that a completely homogeneous distribution of the fissionable materials be obtained. The ideal solution is to use uranium and plutonium as mixed crystals. At the same time it is necessary, especially for ecological and also economic reasons, to recover as much of the plutonium formed or employed as possible from the burned-up fuel assemblies. To this end, high solubility of the fuel employed in nitric acid is necessary, because the use of hydrofluoric acid as the solvent, which is required if the solubility is low, creates problems due to the highly corrosive character of such media. In the interest of plutonium processing for the manufacture of nuclear fuel, high solubility of all products obtained in nitric is also desirable. This desired solubility, however, can be achieved only if uranium and plutonium are present in mixed crystal form.
The chemical attempts for solving the problems up to now have been the joint precipitation of ADU (ammonium diuranate) and Pu(OH).sub.4. However, the product so obtained in the precipitate is not mixed crystals but only a thorough dispersion of the plutonium precipitate in an ADU matrix. With this method, an agglomeration of Pu(OH).sub.4 and the formation of product components insoluble in nitric acid can not always be avoided, however. Furthermore, the separation of impurities by this process is very incomplete.
In another precipitation process for uranium and plutonium of the peroxide precipitation type, the precipitate can be filtered adequately only at low temperatures (.ltoreq.+6.degree. C.) and has a tendency toward explosive decomposition under certain conditions. The (U/Pu)O.sub.2 powder which can be made from the precipitate has poor sintering properties and therefore requires very extensive further processing.
In these mentioned joint precipitation processes for uranium and plutonium, which have been tried on a technical scale, these substances are present in different oxidation stages, so that the ideal precipitation conditions for both elements are distinctly different. This means that a compromise must always be made between the chemical behavior of the two elements at the different oxidation stages.
In the journal "Radiochimica Acta", vol. 11, year '69, pages 123 to 127, the proposal was made in an article by C. Keller to obtain carbonate complexes by reducing uranium and plutonium to the 4-valent oxidation stage in a soda solution, which, after precipitation, can be annealed to form a homogeneous (U/Pu)O.sub.2 nuclear fuel that can be sintered. The reduction was performed electrolytically. These investigations, however, never got beyond the laboratory stage and have been abandoned.
All attempts to solve the problems of homogeneity and solubility of plutonium-containing nuclear fuels by mechanical means involve process-wise very expensive repeated milling and sintering steps. With milling processes, one must furthermore always reckon with abrasion in the mills, which leads to powder highly loaded with impurities.